This Laboratory seeks to elucidate the mechanisms that control the assembly of macromolecular complexes, with particular emphasis on their functional rationale. Over the past year, a major effort has been directed towards enhancing the resolution of three-dimensional density maps of icosahedral virus capsids calculated from cryo-electron micrographs. Thus, we have characterized the events whereby limited proteolysis of the major capsid protein of bacteriophage HK97 induces the stabilizing expansion of the capsid. The excised domain resides on the inner surface of the hexons and pentons, and its removal permits the prohead to maturation transformation to proceed. This transition facilitates the formation of covalent cross-links between neighboring subunits. (ii) In bovine papillomavirus, the analysis has been extended to a resolution of better than 1nm, revealing a wealth of detail concerning the structures of the pentameric capsomers, and disclosing the possible location of the minor capsid protein, L2. (iii) Capsids of herpes simplex virus assembled from proteins synthesized in insect cells from baculovirus expression vectors have been found to be structurally authentic. The 12kDa VP26 protein has been shown to bind to the major capsid protein VP5 only in its hexon state, not in its penton state. From image analysis of negatively stained specimens of the ClpAP energy-dependent protease, the oligomeric natures of the subcomplexes have been defined, as has their mode of interaction. ClpP, the protease, consists of two 7-fold rings, and ClpA, the ATP-hydrolyzing component, forms hexameric rings. In active complexes, they stack axially, resulting in a symmetry mismatch that may have connotations of mutual rotational movement. Progress has also been made on the assembly of cornified cell envelopes of terminally differentiated keratinocytes, and on the domainal organization of bacteriophage tail-fiber proteins.